1. Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens which is built in image pickup devices mounted in increasingly compact and low-profile smartphones and mobile phones, PDAs (Personal Digital Assistants), game consoles, information terminals such as PCs, and home appliances with a camera function.
2. Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, many kinds of home appliances with a camera function which provide high consumer convenience have been introduced into the market. It is expected that in the future the demand for home appliances or information terminals combined with a camera function will grow and products which meet the demand will be developed at an accelerated pace.
The imaging lens built in such products is strongly expected to not only provide high resolution to cope with an increase in the number of pixels but also be compact and low-profile enough to meet the trend toward low-profile products and offer high brightness and a wide field of view to capture an image of an object over a wide range.
However, when an imaging lens meets all the needs for a low-profile design, a low F-value, and a wide field of view, the following problem often arises: it is difficult to correct aberrations especially in the peripheral area of the image, and high optical performance cannot be ensured throughout the image. For this reason, it is difficult to provide an imaging lens which meets the needs for a low-profile design, a low F-value, and a wide field of view and at the same time delivers high resolution performance.
Conventionally, for example, the imaging lenses described in JP-A-2011-141396 (Patent Document 1) and JP-A-2012-103717 (Patent Document 2) are known as imaging lenses designed to deliver high resolution performance.
Patent Document 1 discloses an imaging lens which includes, in order from an object side, a first lens with positive refractive power, a second lens with negative refractive power, a biconvex third lens, a meniscus fourth lens having a convex surface on an image side, a fifth lens in which negative refractive power gradually decreases in the direction from the center toward the peripheral portion and changes to positive refractive power in the peripheral portion.
Patent Document 2 discloses a single-focus optical system which includes, in order from an object side, a first group composed of a first lens with positive refractive power having a convex surface on the object side and a second lens with negative refractive power having an aspheric surface and a concave surface on an image side, a second group composed of a third lens having an aspheric surface, and a third group composed of a fourth lens having a convex surface on the image side and a fifth lens with negative refractive power having an aspheric surface with inflection points and a concave surface on the image side.
The imaging lens described in Patent Document 1 is a relatively compact imaging lens which provides high brightness with an F-value of about 2.0 and corrects aberrations properly. However, its field of view is about 60 degrees and the ratio of total track length to maximum image height (TTL/2ih) is approximately 1.0, which is not enough to meet the recent demand for a wide field of view and low-profileness.
The imaging lens described in Patent Document 2 provides a field of view of about 76 degrees and the ratio of total track length to maximum image height (TTL/2ih) is in the range from 0.8 to 0.9. Thus it is low-profile and provides a relatively wide field of view and corrects various aberrations properly. However, its F-value is about 2.8, so its brightness is not sufficient to cope with a compact high-pixel image sensor. Also, when the imaging lens is designed to provide a wider field of view and a lower profile with high brightness, the problem that aberrations in the peripheral area are hard to correct must be solved.